The present invention generally relates to image processing apparatuses, and more particularly to an image processing apparatus which subjects an image data of a document image to a processing including enlargement or reduction of the document image. Examples of the image processing apparatus are a digital copying machine, a facsimile machine, a filing apparatus, and a computer aided design (CAD) input apparatus.
Generally, when reading a document image, an image reading device such as a charge coupled device (CCD) image sensor reads the document image in minute regions (picture elements) through an optical system including lenses and the like, and converts the density read from each minute region into an electrical signal. The document image is scanned by relatively moving one of the document and the image reading device with respect to the other, and as a result, an image signal describing the entire document image is obtained. However, since the document image is read through the optical system, the quality including contrast and resolution of the image described by the image signal is deteriorated when compared to the quality of the original document image. This deterioration in the quality of the image is generally described by a modulation transfer function (MTF) value. In actual practice, the deterioration in the quality of the image is observed as a fading of the image, for example, and the deterioration in the quality of the image causes undesirable effects when making a character recognition and the like.
Accordingly, a so-called MTF correcting circuit is conventionally provided in this kind of an image reading device so as to compensate for the deterioration in the quality of the image and improve the resolution of the image. The MTF correcting circuit is a kind of a spatial filter which has a function of emphasizing high-frequency components so as to emphasize edges of the image or character. The filter characteristic of the MTF correcting circuit is fixed based on the MTF characteristic of the optical system employed by the image reading device.
On the other hand, in a digital copying machine, a facsimile machine, a filing apparatus and the like, there is often a demand to obtain an output image which is enlarged or reduced with respect to an input image (original document image). Hence, such apparatuses are provided with a function of varying the size of the output image with respect to the input image. But the following problems occur when this function is used.
Firstly, a portion of the input image drops out in the enlarged or reduced image when compared with a case where the input image is outputted in a full size (with a magnification of 100%), that is, the size of the output image is identical to that of the input image.
Secondly, a noise such as a dot which does not appear in the output image when the input image is outputted in the full size becomes conspicuous when the output image is obtained by enlarging or reducing the input image.